This invention relates generally to equipment for production of articles from thermoplastic article preforms. In particular, this invention relates to apparatus for heating thermoplastic preforms in molding equipment known as the reheat and blow type.
In reheat and blow type equipment, article preforms are loaded into a conveyor, transferred through an oven wherein the preform temperature is raised to the molecular orientation temperature, advanced to a blow molding station, and biaxially expanded into finished articles within mold cavities at the blow molding station by injection of an expansion fluid. The finished articles are then ejected for collection.
Because the thermoplastic material used is a poor conductor of heat, preform heating constitutes the most time consuming operation of the overall production process. Both convection and radiant heating suffer the same limiting factor, that is, the elevation of material temperature at the preform surface above the temperature within the preform wall. Excessive heating at the surface can result in material crystallization which, when extreme, prevents normal expansion and, in all cases, degrades desired characteristics of the finished article. Furthermore, both convection and radiant heating result in expenditures of considerably larger quantities of energy than that necessary only to raise the temperatures of the preform material to the desired level. Applicants have devised an apparatus for dielectric heating which overcomes the foresaid disadvantages of convection and radiant heating regarding product quality and energy inefficiencies. Dielectric heating effects material self-heating by inducing motion of inherently polarized molecules within the material by subjecting the material to an alternating electric field. Since these alternating fields are generally operated in the radio frequency (r-f) range, suitable control of a radio frequency generator for use in production blow molding machinery is required and has hitherto been unknown.
It is therefore one object of this invention to provide an apparatus for controlled dielectric heating of thermoplastic article preforms in article molding equipment of the reheat and blow type.
It is a further object of this invention to provide an apparatus for selective time or current controlled dielectric heating of thermoplastic preforms in article molding equipment of the reheat and blow type.
It is a still further object of this invention to provide a controlled dielectric heating apparatus responsive to the anticipated electrical load for selective time or current controlled dielectric heating of thermoplastic preforms in article molding equipment of the reheat and blow type.
Further objects and advantages of the present invention shall become apparent from the following description.